Research On Steam-injection Hot-pressing Technology And Bonding Mechanism Of Non-wood Plant Binderless Particleboard

Non-wood plant resources such as bamboo and crop straw have its huge storage in China, the added value of non-wood plant resources and the use efficiency of agricultural resources was improved by making full use of those resources, in this way, the peasants' income was increased, meanwhile, environmental pollution problem caused by crop straw burning can be solved, human living environment was improved. The main component of bamboo and crop straw were lignin, cellulose, hemicellulose and some ash contents, the chemistry component and structure of those non-wood plant resources is similar to timber, those non-wood plant resource can be used in the production of binderless wood based panel. Steam-injection pressing is a kind of high efficient hot pressing process, in the process of which hot pressing cycle was shortened obviously, the mechanical strength and dimensional stability of binderless wood based panel was improved by the effect of hydrothermal synergistic.This paper analyze the preparation feasibility of binderless particleboard with ramie, tabacco stem and bamboo, the binderless particleboard of ramie, tabacco stem and bamboo were prepared by steam-injection pressing process respectively. The effect on static bending strength, elastic modulus, internal bond strength and thickness swelling of binderless particleboard made by board density, steam pressure and steam injection time were studied. The research has showed that:(1) the effect made by density on the four physical and mechanical properties is obvious, with the density of three different board increased, static bending strength, elastic modulus and internal bond strength were increased obviously, but thickness swelling was firstly increased and then decreased;(2) when the steam pressure is1.0Mpa, the effect on static bending strength and internal bond strength made by steam injection time was obvious, with the steam injection time increased, the static bending strength and internal bond strength was firstly increased and then decreased; when the steam pressure is0.6Mpa, the effect on static bending strength and internal bond strength made by steam injection time was not obvious, when the steam pressure is1.0Mpa or0.6Mpa, the effect on elastic modulus made by steam injection time was obvious, with the steam injection time increased, the elastic modulus of the three different board were increased; when the steam pressure is1.0Mpa and the steam injection time is7min or10min, the elastic modulus, static bending strength and internal bond strength value of the three different board were relatively high; when the steam pressure is1.OMpa or0.6Mpa, the effect on thickness swelling of the three different board made by steam injection time is relatively obvious, with the steam injection time increased, the thickness swelling was decreased; the thickness swelling of the three different binderless particleboard which is prepared by steam injection pressing process meet the standard of GB/T4897.2-2003AND JISA5908;(3) the static bending strength, elastic modulus and internal bond strength of the three different binderless particleboard prepared by steam injection pressing process is obviously higher than that of ordinary hot pressing process. But the thickness swelling is obviously lower than that of ordinary hot pressing under the some condition that the density is0.8g/cm3.The chemical constituents, interface functional groups and cellulose crystallinity variation of the three different binderless particleboard were analyzed by the method of HPLC, FTIR, XPS, CP/MAS13C-NMR, XRD, TG, specific surface area and chemical constituents, non-adhesive bonding mechanisms of the three different binderless particleboard were revealed. The study has showed that there are intense chemical changes in the inner slab by the synergistic effect of hydrothermal in the process of steam injection pressing, the self-adhesive materials were produced in the inner slab:the chemical constituent analysis has showed that the hot water, alcohol benzene extract and lignin content of bamboo, ramie, tabacco stem was obviously increased, the content of hemicellulose, holocellulose and a-cellulose were obviously decreased after steam injection pressing, hemicellulose and some cellulose with low polymerization degree were degraded into monosaccharide and oligosaccharide, meanwhile, the dehydration reaction of degradation products from hemicellulose and a few low polymerization degree cellulose produced furfural and furfuryl alcohol which can react with lignin and its degradation products and produce " lignin-adhesive" which is similar to phenol formaldehyde resin adhesive (PF); the analysis results of HPLC has showed that the hydrolysis of hemicellulose under the synergistic effect of hydrothermal produced oligosaccharides and monosaccharide. The carboxyl from the decarboxylation reaction of oligosaccharides and monosaccharide produced organic acids such as formic acid and acetic acid, the dehydration of partial monosaccharide was converted to furfural. Those organic acids promoted the hydrolysis of hemicellulose and made the hemicellulose hydrolyzed by its own acids. In the process of steam injection pressing, the hydrolysis effect of was more intense, hemicellulose was degraded in a more great extent, which produced more free sugars, organic acids and furfural. The analysis results of specific surface and pore have showed that the hot water extract' residue from the three different materials after steam injection pressing formed more micro pore, which formed more mesopore and macropore by linking together and made the specific surface greatest, the degradation of hemicellulose in the process of steam injection pressing produced more low molecular weight compounds which is soluble in hot water. The analysis results of FTIR have showed that the amount of active hydroxyl in the three different particles was increased obviously by steam injection pressing; the synergistic effect of hydrothermal made hemicellulose hydrolyzed in a more great extent, the polymerization of degradation products produced furfural homopolymer. The analysis results of XPS have showed that the synergistic effect of hydrothermal made partial hemicellulose and a few cellulose degraded more easily, the degradation produced monosaccharide and oligosaccharide, those degradation products was dehydrated and produced furfural compounds, those furfural compounds reacted with lignin and produced a resin polymer which is similar to adhesive; compared to the materials before hot pressing, the content of hemicellulose and cellulose which have hydroxyl in its surface was decreased obviously, the content of hemicellulose and lignin which have carboxyl was hardly changed, the content of hemicellulose and extract which have ester group and carboxyl was increased obviously. The analysis results of CP/MAS13C-NMR have showed that the C1peak of cellulose and the C3, C4, C5peak of lignin from the raw materials, ordinary hot pressing materials and steam injection pressing materials was nearly the same, which showed that hot pressing has a little effect on cellulose and lignin; the degradation speed of hemicellulose was accelerated in the process of steam injection pressing, which made the relative content of hemicelloluse was decreased. Compared to the materials before hot pressing, in the spectra of materials after steam injection pressing, the characteristic peak height of acetyl from hemocellulose near21,72and C2, C3, C5from hemicellulose near72were decreased obviously, the peak tend to gentle. The analysis results of XRD have showed that the synergistic effect of hydrothermal in the process of steam injection pressing was more intense than that of ordinary hot pressing, the hemicellulose was hydrolyzed in a more great extent than that of ordinary hot pressing, which made the non-crystalline region extracted and made the microfibril'hydroxy of crystalline region surface and non-crystalline region exposed, those exposed hydroxy formed hydrogen bond which made non-crystalline region recrystallized, the relative crystallinity of fibers was increased. The analysis results of TG have showed that the TG curve changing trend of the first weight loss stage for the three different materials and that of the third weight loss stage was nearly the same, but the second weight loss stage of TG curve was obviously different. Compared to the materials before hot pressing, the pyrolysis temperature of the second weight loss stage in the process of steam injection pressing moved forward relatively, the slope of TG curve was bigger, which showed that the synergistic effect of hydrothermal in the process of steam injection pressing made more hemicellulose and cellulose degraded and produced some low molecular weight carbohydrate such as low molecular weight sugar which is easy to pyrolysis; the weight loss rate of samples in the third stage was minimum, which explained that in the process of steam injection pressing the furfural from hemicellulose'pyrolysis reacted with lignin and produced some components for adhesion which have higher pyrolysis temperature.The possible mechanism for steam injection molding of non-wood plants binderless particleboard was that:(1) the synergistic effect of hydrothermal made the amount of hydroxyl increased, the hydrogen atom from active hydroxyl combined oxygen atom with its main valence chemical bond and combined the hydrogen atom from near cellulose molecule'hydroxyl which have strong electronegativity with negative-valence chemical bond by forming hydrogen bond (O-H…H).(2)the resinification of furfural which is the degradation product of hemicellulose, furfural can produce furfuryl alcohol with the catalysis of furoic acid, the polycondensation of furfuryl alcohol, with the catalysis of organic acids such as formic acid and acetic acid, can produce condensation furan resins which is helpful to self-adhesion for particle board;(3) under the condition of high temperature and catalysis, the polycondensation between furfural compounds from the hydrolysis of xylan,5-hydroxymethyl-2-furfural from the hydrolysis of fibers and lignin which has phenolic hydroxyl produced a kind of condensation polymer which is similar to phenolic resin;(4) the degradation of partial lignin can produce lignin debris under the effect of hydrothermal. There is chance for the polycondensation between those activated debris and lignin. The degradation of carbohydrate such as hemicellulose made the degraded and activated lignin exposed, which is helpful to the polycondensation of lignin and have the effect of adhesion among those debris.